Mannerism
Style between the Renaissance and the Baroque.
What it means
Mannerism designates a style of the late sixteenth and early seventeenth centuries that stands between the Renaissance and the Baroque. The term comes from art history and is transferred to music; what is meant is an art that deliberately oversteps traditional rules, places effect and expression above balance, and in doing so reaches for sought-after, surprising solutions. Like every period designation it is a later attribution and is used differently in scholarship.
Musical use
In vocal music this shows itself in extensive chromaticism, in unusual chord progressions, in stark contrasts, and in a depiction of the text that goes to the edge of intelligibility. Late madrigals of the sixteenth century contain harmonic turns that appear again comparably only centuries later. Such music was not intended for a broad public but for connoisseurs — which fits the conception of the time of a music for initiates. From here a path leads to monody and to the early Baroque, though the connecting lines are not straight ones.
Performance practice
Such music demands time and calm: practise chromatic passages slowly as a chord progression and work on pure intonation, because otherwise the effect arises from impurity rather than from the harmony. Clarify text and meaning precisely, for the unusual turns usually stand at key words. Sing with moderate vibrato, so that the sonorities remain audible. Decide on solo or choral scoring; in dense chromaticism, solo scoring is often clearer.
Bear in mind that the term is used inconsistently in scholarship and stands for very different phenomena: check what a given text means by it before adopting it. For performance, the substance matters more than the name: such music demands time, pure intonation, and precise knowledge of the text. Practise chromatic passages slowly as a chord progression. With Gesualdo, note that chromatic passages need pure intonation...
Check with editions whether accidentals have been added, because alteration in sources of this period is often not written out.